Three-phase slurry reactors are known in the art, especially for carrying out highly exothermic, catalytic reactions. These reactors have a liquid phase in which solid catalyst particles are dispersed or held in suspension by a gas phase bubbling through the liquid phase. These reactors provide improved heat transfer characteristics for the exothermic reaction, and the bubbling gas provides essentially all of the energy necessary for maintaining the catalyst dispersed in the liquid phase. Stirring or agitation by mechanical means is sometimes used, while additionally a slurry or liquid recycle may be in operation. These bubble column reactors usually comprise a shell-type housing in which a multiplicity of vertically arranged or spirally wound tubes is contained, the tubes being filled with a heat transfer medium, e.g. water and/or steam, which absorbs the heat generated by the exothermic reaction. Usually the reactor comprises a free-board zone located above the slurry zone, which zone contains substantially no slurry, but primarily gaseous products and/or reactants. See for some general literature about three phase slurry reactors Gas-liquid-solid fluidization engineering, L.-S. Fan, Butterworth, Stoneham (1989), and Chemical Reaction Engineering, O. Levenspiel, Wiley and Sons, New York (1972).
The synthesis of hydrocarbons from synthesis gas, i.e. a mixture of hydrogen and carbon monoxide, is well known in the art as the Fischer-Tropsch hydrocarbon synthesis. The reaction is carried out in the presence of a catalyst, usually a group VIII metal catalyst supported on a catalyst carrier. The Group VIII metal is preferably chosen from iron, nickel, cobalt and/or ruthenium, more preferably iron or cobalt. The catalyst carrier is suitably an inorganic refractory oxide, preferably alumina, silica, titania, zirconia or mixtures thereof. Most of the hydrocarbons produced in the Fischer Tropsch reaction are usually in the liquid state under reaction conditions. Preferably heavy hydrocarbons are made, especially C12 and higher, more especially C20 and higher, although also hydrocarbons are produced which are gaseous under the reaction conditions. Further, water is produced, which is mainly present in the gaseous phase at the reaction conditions.
The Fisher-Tropsch reaction may be carried out in a fixed bed multi-tubular reactor or in a fixed bed comprising spirally wound cooling tubes, but can, in view of a more efficient heat transfer, also be carried out in a three phase slurry reactor.
A number of ways have been proposed to separate liquid, especially liquid hydrocarbons reaction products produced in a Fischer Tropsch reaction, from the slurry in a three phase slurry reactor.
Thus, European patent application 609 079 describes a slurry bubble column containing a slurry bed of catalyst particles suspended in a liquid. A filtration zone is located in the slurry bed, in particular close to the upper surface of the slurry bed. The filtration zone typically comprises a plurality of filter elements. The filter elements are typically of elongated cylindrical form and comprise a cylindrical filtering medium enclosing a filtrate collection zone. The filtration results in the formation of a cake, which is removed by back flushing. No indications are given which avoid the building of a cake layer.
European patent application 592 176, describes a filtration zone consisting of a tube sheet holding filter cartridges. The tube sheet defines the upper surface of the slurry bed. No specific indications are given which avoid the building of a cake layer.
International (PCT) application No. 94/16807 describes a filtration zone surrounding the slurry bed. No cake build-up is observed because a very low mean pressure differential is used over the filter elements. A critical value of 6 mbar is mentioned in the description.
UK patent application 2 281 224 discloses a reactor containing a plurality of reaction tubes arranged to accommodate the slurry bed. The upper part of each contains a filter element to separate hydrocarbon product slurry, and a top part of increased diameter, often referred to as a disengagement zone, to separate gas from the slurry. No cake build-up is observed because a very low mean pressure differential is used over the filter elements. A critical value of 6 mbar is mentioned in the description.
U.S. Pat. No. 5,324,335 describes the preparation of hydrocarbons using an (unsupported) iron catalyst. To avoid the continuous increase in slurry height in the reactor vessel, wax is separated from the slurry using a cross-flow filter located outside the reactor vessel. Filter cake is regularly removed by pressurising the filtered wax on the shell side of the filter with an inert gas to bump the cake into the slurry stream.
German patent 3,245,318 describes a process for separating a liquid product stream from a slurry, by cross-flow filtration, which is carried out at substantially reactor pressure, but outside the reactor. Regular back flushing of the filter medium by reversal of the pressure over the filter element is necessary.
A problem in almost all the systems described above is the build-up of a (thick) filter cake. Only at extremely low pressure drops (and corresponding extremely low filtration rates) cake building may be substantially absent. A growing layer of cake decreases the filtration rate, and therefore needs to be removed in order to maintain an acceptable filtration rate. Many ways to remove the filter cake have been described, for instance by using mass forces (e.g. by using a centrifuge), mechanical cake removal (scrapers, doctor blades etc.), reverse flow and vibration.
It would be useful to carry out the Fischer-Tropsch hydrocarbon synthesis in a three phase slurry reactor in such a way that no filter cake is built up on the filter element or a thin, stable cake layer only is built up which layer does not hamper the filtration process. In this way continuous processing would be possible for 1000 hours and more without the need to remove filter cake.